The invention in this case relates to a method of reducing the electromigration of aluminum or aluminum alloy conductors in semiconductor devices.
A frequent cause of failure in aluminum or aluminum alloy conductors employed in high density integrated circuits is found to be electromigration. Electromigration occurs particularly under the conditions of high DC current and high temperature conditions, which conditions occur very frequently in the operation of semiconductor devices. Under these conditions, aluminum metal is transported by the current passing through the aluminum containing conductors, thereby causing the aluminum to form undesired voids and undesired excess deposits. As a result, undesired resistance in these devices is increased to such an extent that an excess amount of electrical and/or heating resistance results, leading to premature failure of such devices.
It is known that electromigration in polycrystalline aluminum conductors essentially proceeds along grain boundaries. These grain boundaries, which are disordered regions between poly-crystalline grains, are characterized by a lower activation energy for diffusion than that of the bulk of the crystalline grain &lt;111&gt;.
Based on this information, a method is described in Pierce, U.S. Pat. No. 4,352,239 in which electromigration is significantly suppressed by a method insuring that the grain boundaries do not extend along the length of a conducting line, but rather extend directly across it.
In this method the grains of the film are formed to be at least as long as the width of the conducting lines and comparable to the length of these lines. As taught by the Pierce patent, unless the average grain size is in fact comparable to the conductor width, there is a high probability of a grain boundary extending for a sufficient distance along the length of the conducting line for electromigration to occur and resulting in failure of the semiconductor device.
Electromigration of aluminum polycrystalline conductors in semiconductor devices is significantly reduced by the method of the Pierce patent. This is achieved by forming the conductors under conditions such that the average grain size grows larger than the width of the conductors and then heat treating the conductors. As a result, grain boundaries are formed which tend to orient themselves across the conductors while the growth of grain boundaries extending along the length of the conductors is virtually stopped.
The resultant orientation of the grain boundaries which is commonly called a "bamboo" structure is found to significantly suppress electromigration in aluminum alloy polycrystalline conductors.
The van de Ven et al U.S. Pat. No. 4,566,177 also shows a method of significantly increasing the electromigration resistance of aluminum alloy conductors in semiconductor devices by the formation of a "bamboo" structure. According to the method of this patent, such a structure is formed by rapidly heating and cooling the conductors, specifically subjecting the conductors to a heat-cool cycle with a peak temperature of 520.degree.-580.degree. C. and as a minimum temperature, ambient temperature, and with a cycle time of about 5 to 30 seconds. The van de Ven patent shows that such cycles may be readily achieved with the use of high intensity CW visual light lamps.